The cooling of a stator heat shield is a challenging task. The heat shields are exposed to the hot and aggressive gases of the hot gas path in the gas turbine. Film cooling of the hot gas exposed surface of the heat shield is not possible at least at those areas of the surface that are arranged opposite to the rotating blade tips. This is for two reasons. Firstly, the complex flow field in the gap between the heat shield and the blade tip does not allow the formation of a cooling film over the surface of this component. Secondly, in case of rubbing events the cooling hole openings are often closed by this event thus preventing the exit of sufficient cooling medium for a reliable film formation with the consequence of overheating the heat shield element. In order to mitigate this risk the clearance between the blade tip and the heat shield must be increased. Currently impingement cooling methods with cooling air ejected at the side faces of the component are a widely-used solution for cooling stator heat shields. WO 2010/009997 discloses a gas turbine with stator heat shields that are cooled by means of impingement cooling in which a cooling medium under pressure, especially cooling air, from an outer annular cavity flows via perforated impingement cooling plates into impingement cooling cavities of the heat shield segment and cools the hot gas path limiting wall of the heat shield. Through ejection holes at the side faces of the heat shield the used cooling medium is ejected into the hot gas path.
According to the patent application CA 2644099 an impingement cooling structure comprises a plurality of heat shield elements connected to each other in the circumferential direction so as to form a ring-shaped shroud surrounding the hot gas path and a shroud cover installed on the radially outer surface to form a hollow cavity therebetween. Said cover has impingement holes that communicate with the cavity and perform impingement cooling of the radially inner wall of the heat shield by jetting cooling air onto its surface inside the cavity. Holed fins divide the cavity into sub-cavities. The cooling air flows through cooling holes in the fins through the fins from a first sub-cavity into a second sub-cavity. Increasing hot gas temperatures require to go down with the wall thickness of the hot gas exposed components to bring down the metal temperatures to acceptable levels. Furthermore, efficiency requirements of modern gas turbines require small clearances between the tips of the rotating blades and the heat shield. However this requirement compromises the design of these elements and their manufacturing that becomes more and more sophisticated and consequently more expensive, and the requirements of rub resistance of the hot gas exposed surfaces, because thin walls increase the risk of damages in case of a rub event.
Patent application WO 2004/035992 discloses a cooled component of the hot gas path of a gas turbine, e.g. a wall segment. The wall segment comprises a plurality of parallel cooling channels for a cooling medium. The inner surfaces of the cooling channels are equipped with projecting elements of specific shapes and dimensions to generate a turbulent flow next to the wall with the effect of an increased heat transfer.
Document DE 4443864 teaches a cooled wall part of a gas turbine having a plurality of separate convectively cooled longitudinal cooling ducts running near the inner wall and parallel thereto, adjacent longitudinal cooling ducts being connected to one another in each case via intermediate ribs. There is provided at the downstream end of the longitudinal cooling ducts a deflecting device which is connected to at least one backflow cooling duct which is arranged near the outer wall in the wall part and from which a plurality of small tubes extend to the inner wall of the cooled wall part and are arranged in the intermediate ribs branch off. By means of this wall part, the cooling medium can be put to multiple use for cooling (convective, effusion, film cooling).
DE 69601029 discloses a heat shield segment for a gas turbine, said segment including a first surface, a back side disposed opposite of the first surface, a pair of axial edges defining a leading edge and a trailing edge, first retaining means adjacent the leading edge and extending from the back side, second retaining means adjacent the trailing edge and extending from the back side, and a serpentine channel including an outer passage extending along one of the edges and outward of the retaining means extending adjacent that edge, an inner passage being inward of the outer passage and a bend passage which extends between the outer passage and the inner passage to place the inner passage in fluid communication with the outer passage, a purge hole which extends from the bend passage to the exterior of the shroud segment to discharge cooling fluid from the bend passage, and a duct extending to the inner passage from a location inward of the adjacent retaining means, the duct permitting fluid communication between the back side of the shroud segment and the serpentine channel such that a portion of the cooling fluid injected onto the back side flows through the serpentine channel, wherein cooling fluid drawn toward the purge hole under operative conditions blocks separation of the cooling fluid in the bend passage.
EP 1517008 relates to cooling arrangement for a coated wall in the hot gas path of a gas turbine based on a network of cooling channels. A gas turbine wall includes a metal substrate having front and back surfaces. A thermal barrier coating is bonded atop the front surface. A network of flow channels is laminated between the substrate and the coating for carrying an air coolant therebetween for cooling the thermal barrier coating.
To ensure sufficient emergency lifetime of the heat shield either the hot gas exposed wall must be designed with a sufficient thickness or the clearance between the blade tips and the stator heat shield must be increased in a way that rubbing contacts during transient operation conditions are excluded. However, this compromises the cooling efficiency in a negative manner.